Fix panic in D208 with multibyte indent (#9147)
Fix #9080
Example, where `[]` is a 2 byte non-breaking space:
```
def f():
""" Docstring header
^^^^ Real indentation is 4 chars
docstring body, over-indented
^^^^^^ Over-indentation is 6 - 4 = 2 chars due to this line
[] [] docstring body 2, further indented
^^^^^ We take these 4 chars/5 bytes to match the docstring ...
^^^ ... and these 2 chars/3 bytes to remove the `over_indented_size` ...
^^ ... but preserve this real indent
```
